A semiconductor device formed from silicon carbide (i.e., SiC) single crystal is expected as a power device to replace a silicon device. To form a SiC power device having high performance, it is required to reduce leak current in the SiC substrate. The leak current in the SiC substrate is caused by a defect such as a screw dislocation in the SiC single crystal.
A conventional method for forming the SiC single crystal is a C-surface crystal growth method. In the C-surface crystal growth method, a SiC seed crystal has a growth surface, which is the C-surface or a surface having an offset angle in a range of 10 degrees (i.e., 10°) from the C-surface. The offset angle of the growth surface is determined from the C-surface. The growth surface of the SiC seed crystal exposes outside so that the SiC single crystal is grown on the growth surface by a sublimation recrystallization method or the like. However, when the C-surface is used for the growth surface of the seed crystal, and the SiC single crystal is grown along with a <0001> direction, many micro pipe defects or screw dislocations are generated along with the <0001> direction in the SiC single crystal.
To reduce the defect in the SiC single crystal, a method for manufacturing the SiC single crystal is disclosed in Japanese Patent Application Publication No. H5-262599. In this method, the seed crystal has the growth surface, which tilts by 60° to 120° from the C-surface. The SiC single crystal is grown along with an A-surface orientation direction so that the method provides an A-surface crystal growth method. In the SiC single crystal grown by the A-surface crystal growth method, a small amount of micro pipe defects and screw dislocations are generated. However, as shown in FIG. 17, the SiC single crystal 9 formed by the above method includes many stacking faults 95 parallel to the C-surface. The stacking faults 95 have high density in the SiC single crystal 9. Therefore, electric resistance of the SiC single crystal 9 perpendicular to the C-surface (i.e., parallel to a C-axis of a <0001> direction) becomes larger. Specifically, when electrons cross the stacking fault 95, the resistance becomes larger. Thus, the SiC single crystal 9 having high-density stacking fault 95 can not be used for the SiC power device.
To remove the stacking fault, in a process for manufacturing the SiC single crystal, it is required to use a seed crystal having a growth surface of the C-surface. Specifically, the A-surface grown seed crystal having no screw dislocation substantially is prepared firstly. Then, the A-surface grown seed crystal is cut so that the seed crystal having the growth surface of the C-surface is obtained. Here, the C-surface is exposed outside. Next, this seed crystal is used for the C-surface crystal growth so that the SiC single crystal having no screw dislocation nor stacking fault is obtained. However, in this method, as shown in FIG. 18, since the A-surface grown seed crystal 8 having the C-surface growth surface 81 has no screw dislocation substantially, there is no source for supplying steps when the C-surface crystal growth is performed using the A-surface grown seed crystal 8. The source supplies steps so that the A-surface grown seed crystal 8 is transcribed to a growing crystal 80. Specifically, the source supplies steps so that polymorphism of the A-surface grown seed crystal 8 is transferred to the growing crystal 80. However, there is no source so that the growing crystal 80 includes a heterogeneous polymorphous crystal 87 or a different surface orientation crystal 88, partially. If the heterogeneous polymorphous crystal 87 or the different surface orientation crystal 88 is grown, the SiC single crystal 80 includes many screw dislocation 89 at random. The SiC single crystal 80 having the different surface orientation crystal 88 or the screw dislocation 89 is not used for the device because of leak current.
In view of the above problem, another method for forming the SIC single crystal is disclosed in Japanese Patent Application Publication No. H8-59389. The method is such that the growth surface of the seed crystal includes an artificial convexity or concavity as a singular point. The singular point generates a screw dislocation as the source for supplying steps. Thus, the SiC single crystal is grown. However, the simple convexity or concavity has no screw component so that they may not work as the source for supplying steps for transferring the polymorphism of the A-surface grown seed crystal to the growing crystal. Specifically, when the SiC single crystal is grown by a certain amount, the convexity and concavity changes to the screw dislocation so that the convexity and concavity works as the source for supplying steps. However, in the beginning of the crystal growth, the heterogeneous polymorphous crystal 87 may be generated so that the heterogeneous polymorphous crystal 87 expands whole growing crystal 80. Then, the SiC single crystal may have inferior quality.